The invention relates to a receiving device used in radio communication for transferring a pilot signal and, more particularly to, a receiving device applied to a high-definition television.
Conventionally, there has been such a communication method that transfers a pilot signal so that a receiving device can demodulate a received signal accurately. As well known, the pilot signal is used to detect a carrier wave accurately on the receiving device. For example, the high-definition television broadcasting employed in U.S.A. etc. utilizes this communication method of transferring the pilot signal mentioned above.
Conventional high-definition television is equipped with an oscillator for outputting a signal having a frequency which corresponds to a currently selected channel. A tuner outputs a signal of an intermediate frequency (hereinafter refer to IF signal) which is obtained by tuning a radio wave received through an antenna to an output signal from this oscillator. Then, this IF signal is filtered by a SAW filter (surface acoustic wave filter) within a predetermined frequency band width. The band width of the SAW filter is, for example, 6 MHz.
The IF signal within the predetermined band width filtered at the SAW filter is amplified by an amplifier and then input to a demodulator. The demodulator extracts the pilot signal from thus received signal and demodulates the IF signal by using extracted pilot signal.
However, if the frequency accuracy of the output signal of the oscillator equipped in the receiving device is poor, the frequency of the IF signal output from the tuner is shifted from an appropriate value. If, for example, a frequency of the signal output from the oscillator actually is B despite that the receiving device controls the output signal of the oscillator so that the frequency may be A, a frequency error of the output signal of the oscillator is (A−B). If this frequency error is large, no pilot signal is present in the IF signal within the predetermined band width filtered by the SAW filter or that the pilot signal, even if present, cannot be extracted. This accident leads to such a problem that the signal cannot be received.
Also, the broadcasting station sometimes broadcasts a program with its carrier wave frequency as shifted (i.e., using an offset carrier wave). For example, if there is a broadcasting station broadcasting a program using a carrier wave having a frequency which is close to that in an adjacent area, an offset carrier wave may be used to avoid radio interference with this broadcasting station in U.S.A..
In this case also, no pilot signal is present in the IF signal within a predetermined band width filtered by a SAW filter or that the pilot signal, even if present, cannot be extracted. This accident, therefore, leads to a problem that the signal cannot be received like in the case mentioned above where the frequency error is large.
A technology for solving the problem mentioned above is disclosed in, for example, Japanese Unexamined Patent Publication No. 9-121315. It proposes that a pass-band width of the SAW filter should be shifted in a direction of tuning the signal by the tuner. The configuration thereof, however, is complicated, to increase the costs of the device, so it is not a practical method.